1. Field of the Invention
The present invention relates to a process for producing an optical waveguide, in particular, a flexible polymer optical waveguide, and to a resin injecting device.
2. Description of the Related Art
As a process for producing a polymer optical waveguide, the following processes are suggested: (1) a process of impregnating a film with a monomer, selectively exposing a core portion to light to change a refraction index thereof, and sticking a film thereto (selective polymerization); (2) a process of applying a core layer and a clad layer to a substrate, and forming a clad portion by reactive ion etching (RIE); (3) a process using photolithography, in which an ultraviolet ray-curable resin obtained by adding a photosensitive material to a polymer is imagewise exposed to light and developed (direct exposure); (4) an injection molding process and; (5) a process of applying a core layer and a clad layer to a substrate, and exposing a core portion to light to change a refraction index of the core portion (photo bleaching).
However, the selective polymerization process (1) has a problem with regard to the sticking of the film, and the processes (2) and (3) result in an increase in costs since photolithography is used. The process (4) has a problem with regard to precision of the resultant core diameter, and the process (5) cannot provide a sufficient refraction index difference between the core layer and the clad layer.
At present, practical processes which have superior performance include only the processes (2) and (3). However, these processes also have a problem with regard to costs as described above. Additionally, none of the processes (1) to (5) can be applied to the formation of a polymer optical waveguide on a flexible plastic substrate having a large area.
A process of filling a pattern substrate (clad) in which a pattern of grooves, which are to serve capillaries, is formed with a polymer precursor material for a core, curing the precursor material to form a core layer, and then bonding a flat substrate (clad) onto the core layer is known as a process for producing a polymer optical waveguide. However, in this process, not only the capillary grooves but also the entirety of the narrow space between the pattern substrate and the flat substrate is filled with the polymer precursor material, and, when the polymer precursor material is cured, a thin layer having the same composition as the core layer is formed between the pattern substrate and the flat substrate. Therefore, light leaks out through this thin layer.
As one method of solving this problem, David Heard suggests a method of fixing a pattern substrate in which a pattern of grooves, which are to serve as capillaries, is formed to a flat substrate with a clamping member, sealing a contacting portion between the pattern substrate and the flat substrate with a resin, and then reducing an internal pressure to fill the capillaries with a monomer (diallyl isophthalate) solution, thereby producing a polymer optical waveguide (Japanese Patent Gazette No. 3151364).
This method uses the monomer as the core forming resin material instead of using a polymer precursor material to reduce a viscosity of the filling material and fill the capillaries with the filling material using capillarity, so that no other space than the capillaries is filled with the monomer.
However, because of the use of the monomer as the core forming material in this method, a volume shrinkage ratio of the monomer is large when the monomer is polymerized. Consequently, transmission loss of the polymer optical waveguide becomes large.
This method is also a complicated method, in which the pattern substrate and the flat substrate are fixed to each other with the clamp, and the contacting portion is sealed with the resin. Thus, this method is not suitable for mass production. As a result, a reduction in costs cannot be expected. Moreover, this method cannot be applied to the production of a polymer optical waveguide using, as a clad, a film having a thickness on the order of several millimeters or a thickness of 1 mm or less.
Recently, George M. Whitesides et al. at Harvard University have suggested, as a method for forming a nanostructure and as a soft lithographic process, a method called capillary micromolding. This is a method of using photolithography to form a master substrate, making use of adhesiveness of polydimethylsiloxane (PDMS) and an easily-peelable property thereof to transfer the nanostructure of the master substrate onto a mold made of PDMS, pouring a liquid polymer into this mold by capillarity, and curing the polymer. A detailed description thereof appears in SCIENTIFIC AMERICAN September 2001 (Nikkei Science, 2001, December).
Moreover, a patent for the capillary micromolding method was granted to Kim Enoch et al. of George M. Whitesides' group at Harvard University (U.S. Pat. No. 6,355,198).
However, even if the production process described in this patent is applied to the production of a polymer optical waveguide, it takes much time to form a core portion thereof since a sectional area of the core portion of the optical waveguide is small. Thus, the process is unsuitable for mass production. This process also has a drawback in that when a monomer solution is polymerized, a volume change is caused, so that a shape of the core is also changed, and consequently transmission loss becomes large.
B. Michel et al. of IBM Zurich Laboratory suggest a lithographic technique exhibiting a high resolution by the use of PDMS, and report that this technique provides a resolution on the order of several tens of nanometers. A detailed description thereof appears in IBM J. REV. & DEV. VOL. 45 NO. 5 Sep. 2001.
As described above, the soft lithographic technique using PDMS and the capillary micromolding method are techniques which have been recently drawing attention as nanotechnologies, primarily in the U.S.A, but elsewhere as well.
However, when an optical waveguide is formed by a micromolding method as described above, reduction of the volume shrinkage ratio of a polymer (so that transmission loss is reduced) when the polymer is cured is incompatible with reduction of the viscosity of a filling liquid (the monomer and so forth) in order to attain easy filling. Accordingly, in a case where reduction of transmission loss is preferentially considered, the viscosity of the filling liquid cannot be lowered sufficiently and a filling speed becomes slow. Thus, the mass production of optical waveguides by this method cannot be expected. The micromolding method is carried out on the assumption that a glass or silicon plate is used as a substrate. Thus, use of a flexible film substrate in this method has not been considered in this method.